\begin{tikzpicture}[scale=0.8, >=Stealth,
    every node/.style={fill=white, inner sep=1pt},
]
    \draw (-0.5, 0) -- (14.5, 0);
    \foreach \x in {0, 1, ..., 14} {
        \ifnum \x = 7
            \draw (\x, -0.5) -- (\x, 0.4) node [above=0.1em] {$\x$};
        \else
            \draw (\x, 0) -- (\x, 0.2) node [above=0.2em] {$\x$};
        \fi
    }

    \draw [<->, very thick] (0.5, -1) -- (13.5, -1)
        node [midway] {中性}
        node [pos=0.2, below=0.5em] {酸性增强}
        node [pos=0.8, below=0.5em] {碱性增强};
\end{tikzpicture}

